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Observations of Nontornadic Low-Level Mesocyclones and Attendant Tornadogenesis Failure During VORTEX*

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Observations of Nontornadic Low-Level Mesocyclones and Attendant Tornadogenesis Failure During VORTEX* JULY 1999 NOTES AND CORRESPONDENCE 1693 Observations of Nontornadic Low-Level Mesocyclones and Attendant Tornadogenesis Failure during VORTEX* R. J. TRAPP NOAA/National Severe Storms Laboratory, and Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma 19 February 1998 and 13 July 1998 ABSTRACT Three storms intercepted during the Veri®cation of the Origins of Rotation in Tornadoes Experiment generated a moderate-to-strong mesocyclone within the lowest several hundred meters above the ground and qualitatively appeared capable of tornadogenesis, yet did not produce a tornado. Such novel observations of what is considered ``tornadogenesis failure'' are documented and used to show the insuf®ciency of a low-level mesocyclone for tornadogenesis. Possible modes of failure are discussed. 1. Introduction and in other ways appeared ``primed for tornadogene- sis'' (Brandes 1993), yet did not produce tornadoes. In The Veri®cation of the Origins of Rotation in Tor- other words, existence of a low-level mesocyclone was nadoes Experiment (VORTEX; Rasmussen et al. 1994) an insuf®cient condition for tornadogenesis. One may was conducted during the spring of 1994 and 1995 in argue, then, that theories of low-level mesocyclogenesis the southern Great Plains of the United States. The ob- (e.g., Rotunno and Klemp 1985; Davies-Jones and jectives of this validation experiment were driven by a Brooks 1993; Brooks et al. 1993, 1994) fall short of set of hypotheses (see Rasmussen 1995) that concerned explaining the details of tornadogenesis, as underscored (i) the initiation of tornadic storms; (ii) low-level me- by the idealized modeling results of Trapp and Fiedler socyclogenesis, tornadogenesis, and the role(s) of me- (1995). Hence, another layer of complexity must be ad- soscale and stormscale boundaries in each; and (iii) the dressed by theories of tornadogenesis. dynamics of tornadoes and their associated boundary The aforementioned nontornadic storms are consid- layers. ered examples of ``tornadogenesis failure,'' an arguably VORTEX data presented herein provide additional subjective classi®cation introduced here to provide a observational basis for the sentiment expressed in Da- basis for comparison with tornadic storms and to help vies-Jones and Brooks (1993): ``in perhaps the least clarify tornadogenesis mechanisms. This concept of known process, a tornado develops within the meso- ``failure'' is attributed to Brooks et al. (1993), albeit in cyclone.'' Indeed, several storms intercepted during the context of low-level mesocyclogenesis. These au- VORTEX generated moderate to strong low-level me- thors identi®ed failure modes in which a strong (vertical 1 socyclones that were sustained for a $15 min period, vorticity, z ; 0.01 s21) midlevel and/or low-level me- socyclone neither developed nor persisted in a numer- ically simulated storm; a failure mode was presumed to preclude the formation of a long-lived, signi®cant tor- 1 This approximates the minimum amount of time required to am- 2 plify, through vortex stretching with a constant divergence of 25 3 nado. A proper balance between two opposing factors 1023 s21, vorticity of a strong mesocyclone (z ; 1 3 1022 s21)to that govern rear-¯ank gust-front movement, namely the that of a tornado (z ; 1s21); these vorticity and divergence values environmental in¯ow and the cold-air out¯ow, was are well within the range of observed values (e.g., Lemon and Doswell found to be of critical importance in low-level meso- 1979). cyclogenesis and sustenance [and in the sustenance of the parent storm itself; Weisman and Klemp (1982)]. * A portion of this research was presented at the 28th Conference The VORTEX observations imply a new challenge on Radar Meteorology, Austin, Texas. Corresponding author address: Dr. R. Jeffrey Trapp, NSSL, 1313 Halley Circle, Norman, OK 73069. 2 The gridpoint spacing was too coarse to resolve tornado-scale E-mail: [email protected] motions. q 1999 American Meteorological Society Unauthenticated | Downloaded 09/25/21 10:12 PM UTC 1694 MONTHLY WEATHER REVIEW VOLUME 127 to operational forecasters in their efforts to discriminate, TABLE 1. Approximate distance (km) of each storm's mesocyclone for warning purposes, storms that are likely to produce from the airborne Doppler radar and nearest WSR-88D, at analysis time T. Vertical resolution is estimated for the airborne Doppler radar tornadoes from those that will not. A means to discrim- at the indicated range, assuming an azimuthal sampling interval or inate unambiguously between tornadic and nontornadic sweep-angle resolution of 1.28. Asterisk denotes tornadic case. storms has yet to be demonstrated in the literature, hence Distance to Vertical motivating related research at the National Severe airborne data Distance to Storms Laboratory (NSSL) involving VORTEX data radar spacing Nearest WSR-88D and complementary numerical modeling. This report Case (km) (km) WSR-88D (km) provides the underpinning of some of that work by es- 0429 14 0.29 KFWS 135 tablishing existence of nontornadic low-level mesocy- 0512 10 0.21 KDDC 160 clones and attendant tornadogenesis failure during 0522 16 0.34 KAMA 125 VORTEX. 0529* 9 0.19 KFWS 155 0417* 10 0.21 KTLX 140 A description of the datasets and analysis method 0516* 19 0.40 KDDC 80 used toward this end is provided in section 2. In section 3, evidence to support the classi®cation of failure within three nontornadic storms is presented. A diagnostic comparison with three tornadic storms is used in section observations) and after which low-level vertical vortic- 4 to explore and provide additional clues on possible ity diminishes, is designated as time of tornadogenesis modes of failure. Some preliminary remarks on failure failure. A tornado is de®ned as in the Glossary of Me- modes are discussed in section 5. teorology. Although each instance of tornadogenesis failure is by de®nition a nontornadic event, it is not assumed herein that all nontornadic storms are instances 2. Data description of tornadogenesis failure; ``nontornadic'' carries with it Airborne Doppler radar (ADR) observations, ob- no distinction about the existence of low-level rotation, tained by a helically scanning X-band radar mounted in which, on some scale larger than that of the tornado, is the tail of one of the NOAA P-3 aircrafts, make up the necessary for tornadogenesis. primary data source. When employing either the fore± WSR-88D data are used to identify times of maxi- aft scanning technique or the all±fore, all±aft scanning mum low-level rotational velocity and thus guide which technique, the P-3 is capable of gathering pseudo-dual- ADR ¯ight legs to analyze. Using calculations of ver- Doppler data during a ¯ight leg (Jorgensen et al. 1996). tical vorticity from the pseudo-dual-Doppler synthe- A typical leg lasts ;5 min and is ¯own at a distance sized winds, tornadogenesis failure time is then veri®ed, of ;10±20 km from the storm and at an altitude of ;1 or adjusted if necessary. Additional analyses are pro- km above ground level. The P-3 aircrafts also are duced at ;5 and ;10 min prior to failure time (here- equipped with a lower-fuselage (LF)-mounted C-band inafter denoted as T5 and T10). conventional radar. This radar scans at a rate of two revolutions per min and collects re¯ectivity information 3. Observational evidence of tornadogenesis failure at low elevation angles with respect to the quasi-hori- zontal plane containing the aircraft ¯ight track. The It is prudent to recall ®rst the pioneering efforts of Weather Surveillance Radar-1988 Doppler (WSR-88D) Stout and Huff (1953), Browning and Donaldson nearest to each storm supplements the LF and airborne (1963), Chisholm and Renick (1972), Marwitz (1972), Doppler radars. For reference, approximate distances of and Lemon (1980). These researchers, among others these radars from the mesocyclone center of each storm, [see the reviews by Donaldson (1990) and Burgess and at the time of tornado formation or failure, are listed in Lemon (1990)], introduced supercell storm air¯ow mod- Table 1. Details of the ADR data analysis are provided els and radar-observable characteristics such as pendant in the appendix. and hook echoes from which rotation may be inferred; The criterion for VORTEX case consideration and bounded weak echo regions (BWER) as a means to selection is existence of pseudo-dual-Doppler data on identify regions of strong updrafts; rotation (low-alti- the storm, at least 10±15 min prior to the time of its tude convergence, storm-top divergence) signatures in failure. To provide a basis for comparison with tornadic mean Doppler velocity that depict mesocyclones (up- storms, failure is de®ned to be the lack of tornado for- drafts); and spatial correlations of a mesocyclone sig- mation within a strong (z $ 0.01 s21) low-level me- nature and BWER that indicate a rotating updraft. socyclone whose life cycle is $15 min, and furthermore Such characteristics have been used in descriptions within a storm possessing other characteristics shown of supercell storm life cycles. The stage that heralds to be associated with a transition to a tornadic phase tornado formation is of relevance in the present dis- (Klemp 1987; see section 3). The time of occurrence of cussion. Brandes (1993), for example, described a ``ma- peak low-level vertical vorticity, which in these cases ture stage'' as ``that critical period in storm evolution corresponds to the time at which tornadogenesis appears at which the basic updraft and vertical vorticity patterns imminent (from subjective visual and/or weather radar associated with supercells have evolved and the storm Unauthenticated | Downloaded 09/25/21 10:12 PM UTC JULY 1999 NOTES AND CORRESPONDENCE 1695 is primed for tornadogenesis.'' A rudimentary hook kinematic occlusion of the mesocyclone at time T. Qua- echo, WER or BWER, rainy rear-¯ank downdraft and si-vertical tail radar scans of the three storms depict the associated gust front, and an arc-shaped updraft that presence of a deep, echo-free vault, or BWER, and correlates spatially with vertical vorticity, are qualitative therefore extensive updraft (Fig.
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